1. Field of the Invention
This invention relates to a structure for a variable electronic component, such as variable resistor or variable capacitor, wherein a certain parameter or physical property of the component is adjusted by rotating a rotor relative to a substrate.
2. Description of the Prior Art
A variable resistor of the above-described type is disclosed for example in Japanese Utility Model Application Laid-open No. 2-92903 (Laid-open: Jul. 24, 1990; Inventors: Tamotsu YOSHIMURA et al.; Applicant: Rohm Co., Ltd.). The resistor comprises an insulating substrate formed with a resistor strip, and a metallic rotor member rotatably supported on the substrate. The rotor member has a relatively large central cup recess and an annular flange surrounding the central recess. The annular flange is formed with engaging grooves for engagement with a driver used for turning the rotor member. A part of the annular flange is downwardly deformed to work as a contact portion which comes into pressing and sliding contact with the resistor strip, so that the resistance of the resistor is adjustable by turning the rotor member relative to the substrate.
For automatic assembly of the prior art variable resistor, the rotor member is picked up by a vacuum collet for transfer to the substrate. However, since the flatness of the rotor member is very low due to the presence of the central cup recess, engaging grooves and downwardly deformed contact portion, a large degree of vacuum is required for stably holding the rotor member. Otherwise, the vacuum collet holds the rotor member in an inclined posture, thus frequently failing to properly assemble the resistor.
In view of the above problem, Japanese Utility Model Application Laid-open No. 3-85604 (Laid-open: Aug. 29, 1993; Inventor: Tamotsu YOSHIMURA; Applicant: Rohm Co., Ltd.) proposes the use, in a variable resistor, of a metallic rotor which comprises a cup-form main rotor member and a flat upper plate member integral with the main rotor member via a connecting web which is bent so that the upper plate member overlaps on the main rotor member. The upper plate member has an engaging opening for engagement with a driver. Obviously, the flatness of the upper plate member enables a vacuum collet to hold the rotor reliably at the time of assembling the resistor.
However, the prior art variable resistor described above gives rise to a new problem, as described below.
At the time of performing a parameter adjustment, the rotational force of the driver is first applied to the upper plate member and then transmitted to the main rotor member via the connecting web. However, the friction between the main rotor member and the insulating substrate provides a resistance against rotation of the main rotor member relative to the substrate. Such a frictional resistance causes the connecting web to be elastically deformed at the time of turning the driver (namely, the upper plate member), and the thus deformed connecting web in turn causes the main rotor member to rotate with a slight delay relative to the upper plate member. Further, even after stopping the turning of the driver, the main rotor member still continues to rotate due to the elastic restoration of the deformed connecting web. As a result, it is very difficult to provide a stable and accurate parameter adjustment.
It is conceivable to solve the above problem by increasing the width of the connecting web and/or the thickness of the material metal plate for the rotor, thereby strengthening the connecting web. However, an increase of the width of the connecting web makes it difficult to bend the material metal plate at the connecting web, consequently resulting in a reduction of the productivity (namely, an increase of the production cost). Further, an increase of the thickness of the material metal plate also makes it difficult to press and bend the material metal plate in addition to increasing the material cost.